This project examines mitochondrial functioning in old age and in pathological states in which decreased energy transduction by mitochondria may compromise tissue survival. We have focused this year on mitochondrial functioning in the hearts of rats made diabetic by treatment with streptozotocin and studied in the chronic stage of the disease (6-8 weeks after treatment). We have extended last year's findings on the relative inability to elevate intramitochondrial free Ca2+ ([Ca2+]m) of cardiac myocytes from diabetic rats, when subjected to electrical stimulation. This was studied using indo 1:AM-loaded and Mn2+- quenched individual cardiac myocytes, using fluorescence microscopy. Stimulation (4 Hz) in the presence of elevated superfusate Ca2+ concentration (3.5mM) gave significantly smaller values of [Ca2+]m in myocytes from diabetic animals, when compared with controls. Isolation of cardiac mitochondria from the two groups of animals revealed no differences in the kinetics of Ca2+ uptake or release, however, when studied using metallochromic indicators. It is concluded that the failure to elevate [Ca2+]m in cells from the diabetic animals, which is of a magnitude such that it would be expected to limit the activation by Ca2+ of pyruvate dehydrogenase and the tricarboxylate cycle, derives from a failure of excitation-contraction coupling in this model.